A significant problem in the clinical management of cancer is the identification of an appropriate treatment strategy. For example, the choice of whether a patient is subjected to conventional versus fractionated radiation therapy is often dependent upon the proliferative status of a tumor. The primary measure of proliferative status is the determination of the S-phase fraction of a tumor. This is traditionally determined by flow cytometric measurements of tissue biopsy samples. Patients with tumors exhibiting a high S-phase fraction display a greater likelihood of tumor recurrence and have a higher death rate. These patients, who are predicted to have a poor response to conventional radiation therapy, are often chosen for an accelerated radiation fractionation schedule.
Although S-phase fraction is an objective method for measuring the proliferative status of tumors, there are a number of complications that limit the accuracy of the procedure. For example, in breast cancer, 30–40% of biopsy samples cannot be evaluated by flow cytometric analysis. Furthermore, tissue sampling by biopsy can be problematic since most tumors are heterogeneous, and consist of both proliferative and nonproliferative cells. Therefore, tissue samples obtained from a tumor biopsy may not be representative of the entire tumor cell population.
Imaging procedures that avoid many of the problems associated with traditional procedures include single photon emission computed tomography (SPECT) and positron emission tomography (PET). Unlike flow cytometry of biopsy samples, which sample only a fraction of the tumor, SPECT and PET can image and provide information about an entire tumor.
These imaging techniques have been used in conjunction with radioactive tracer compounds (radiotracers) that possess a high affinity for a protein having abnormal expression in tumor cells. The most prominent example of this approach is the use of radiolabeled monoclonal antibodies possessing a high affinity for tumor-radiolabeled monoclonal antibodies possessing a high affinity for tumor-associated antigens. Although some success has been obtained in this area, a number of complications, including heterogeneity of antigen-containing tumor cells, low tumor uptake, nonspecific radiotracer uptake in adjacent or other nontumor tissues, the presence of circulating antigens that compete with tumor cells for antibody, and the potential immunogenicity of the monoclonal antibody, have limited the general utility of this approach.
Another alternate approach for imaging tumors is the use of radiolabeled small molecules that possess a high affinity for receptors, such as sigma receptors, that are abnormally expressed in tumor cells. Sigma (σ) receptors have been defined as nonopiate, nondopaminergic, and nonphencyclidine receptors based on their ligand binding characteristics. It is believed that sigma receptors exist in at least two distinct subtypes, referred to as sigma-1 (σ1) and sigma-2 (σ2). An alternatively spliced variant of the σ1 receptor, termed σ-R1A, has been cloned and expressed from Jurkat human T lymphocytes. M. E. Ganapathy et al. (J. Pharmacol. Exp. Therap. 289, 251 (1999). When compared to the σ1 gene, this variant has three amino acid (AA) substitutions, a deletion in exon III (AA 118–149) and a loss of σ1 ligand binding activity.
A high density of both sigma-1 and sigma-2 receptor subtypes have been expressed in many human and rodent tumor cell lines. See B. J. Vilner, et al., Cancer Res. 55:408–413 (1995). High levels of sigma receptors have also been reported in human tumor cells and in membrane preparations obtained therefrom. See, e.g., G. E. Thomas, Life Sci. 46:1279–1286 (1990); W. T. Bem, et al., Cancer Res. 51: 6558–6562 (1991); C. S. John, et al., J. Nucl. Med. 37:267P (1996); C. S. John, et al., J. Nuc. Med. 34:2169–2175 (1993); C. S. John, et al., J. Med. Chem. 37:1737–1739 (1994); C. S. John, et al., Life Sci. 56:2385–2392 (1995); C. S. John, et al., J. Nucl. Med. 37:205P (1996). Exemplary tumor cells and cell membranes expressing sigma receptors include brain tumor cells, breast tumor cells, human melanoma cells, non-small cell lung carcinoma cells, and human prostate tumor cells.
U.S. Pat. No. 5,863,766 to Hillman et al, describes the DNA sequence of a human sigma receptor, as well as isolated proteins encoded by the same and expression vectors, host cells, agonists, antibodies and antagonists of the same. U.S. Pat. Nos. 5,919,934 and 5,911,970 to John et al. describe compounds, methods and methods for cancer diagnosis, imaging and therapy, particularly in relation to cancer cells that have a cell surface sigma receptor.
Studies have suggested that σ2 receptors are a biomarker of tumor cell proliferation. See e.g., Mach et al. Cancer Research 57, 156–161 (1997) and Al-Nabulsi et al., British J. Cancer 81, 925–933 (1999). Particularly, σ2 receptors were found to be expressed eight to ten times more in proliferative (P) tumor cells than in quiescent (Q) tumor cells. In a recent study, the σ2 receptor P:Q ratio was about 10.6 in solid tumors and about 9.5 in a tissue culture study. Wheeler et al., Br. J. Cancer 86, 1223–1234 (2000).
In view of the foregoing, sigma receptors are useful as markers in the non-invasive detection and visualization of a wide variety of tumors using single photon emission computed tomography and positron emission tomography technology. Previous reports have demonstrated that sigma receptors may serve as a target for radiotracers that can be used to anatomically image solid tumors. A correlation between sigma receptor density and the proliferative status of tumor cells was suggested in International Patent Application PCTUS97/04403 (claiming priority from U.S. Provisional Application 60/013,717, which is incorporated herein in its entirety). This application describes the discovery that σ2 receptor density correlates with the proliferative status of breast tumor cells, and also describes a non-invasive method to detect cancer cells or to assess the proliferative status of cancer cells which express σ2 receptors, using detectably labeled σ2 ligands. International Application PCT/US00/13834 (claiming priority from U.S. Provisional Application 60/135,274, which is incorporated herein in its entirety), describes methods of determining the proliferative status of cancer cells by determining the ability of proliferative cells to bind σ1 and σ2 ligands, respectively. Specifically, it is suggested that the ratio of σ2 to σ1 density on a cell is an indicator of the proliferative state of the cell. See also Wheeler et al., supra, which is also incorporated herein by reference.
Despite the reported use of detectably labeled σ2 ligands to determine the proliferative status of cancer cells, there remains a need for additional methods and compositions for accurately assessing the proliferative status of cancer cells. A need also remains for non-invasive methods and compositions that are useful for imaging tumors and diagnosing cancer and other disorders of cell proliferation.